1. Field of the Invention
Embodiments of the present invention are directed to phosphor compositions emitting yellow-green light in a wavelength ranging from about 500 to about 560 nm, wherein the emitted light has a decay time of ranging from about 1 ms to about 10 ms. Moreover embodiments of the invention relate to AC operable white light emitting devices and lighting systems utilizing such phosphors.
2. Description of the Related Art
Wavelength conversion methods that use excitation light produced by solid-state light sources, such as laser diodes (LDs) or light emitting diodes (LEDs), and photoluminescence wavelength converting materials, such as phosphors and quantum dots can produce bright light at wavelengths that are different from the wavelengths of the excitation light. In conventional devices, excitation light impinges on a wavelength conversion material which absorbs the excitation light and emits light at a wavelength higher than the wavelength of the excitation light.
It is now common to implement white light sources, such as solid-state white light sources, using photoluminescence wavelength conversion materials. An LED that is capable of generating excitation light with wavelengths in the UV or blue region of the electromagnetic spectrum is used in conjunction with the excitation light source to generate, for example, white light. As taught in U.S. Pat. No. 5,998,925, lighting systems based on white LEDs may include one or more photoluminescence materials (e.g. phosphors), capable of absorbing a portion of the radiation emitted by the radiation emitted by the LED, thereby generating emitted radiation of a different wavelength (e.g. color). Typically, the LED chip or die generates blue light, and the phosphor(s) absorbs a percentage of the blue light, in turn emitting yellow, or a combination of red and green light, green and yellow light, green and orange light, or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor; this produces a product light that appears to the human eye to be nearly or substantially white in character.
A lighting system requires, of course, a source of electrical power. Such electrical power sources may be operate in either a DC (direct current) or AC (alternating current) mode. When a DC drive is used to power the source of the excitation light (e.g. the LED), a relatively continuous current level is maintained in the electrical source current. Therefore, for DC-based lighting applications, the photoluminescence materials used in the wavelength conversion components preferably have decay times of less than a millisecond, so that light from the lighting system can be turned on and off in an immediate fashion, in response to the electrical switch being turned on and off, respectively.
It is also possible that an alternating current (AC) source may be used to drive an LED lighting system. When an AC power supply is used, the electrical current in the circuit forms a wave pattern that “alternates” between two different current levels, where mathematically, the pattern may be described by a sine wave. An LED that is operable with AC current is called an AC LED. A rectifier may be used to provide a doubling of the frequency of the input current driving the AC LED. The rectifier may be implemented in simple rectifying circuitry, without a capacitor or complex integrated circuit (IC) components, whose purpose would have been to obviate the need of an electrolytic smoothing capacitor(s). The reason for avoiding electrolytic smoothing capacitors is that they have a lifetime which is often much less than the life expectancy of the LED chip.
There is a need in the art for phosphor compositions that are optimized for use in AC LED based lighting systems. Such phosphor compositions are configured for long decay times in their photoluminescence emissions; this is so that the decaying light can be used to fill “gaps” in luminescence that otherwise would have occurred without a ling decay phosphor. The “gaps,” or periods of low luminosity are due to the fact that the AC power is cycling to fully on states, through zero (fully off states), to fully on states again in the opposite polarity.